1. Field of the Invention
This invention relates to a method for growing a silicon single crystal using silicon granules.
2. Description of Related Art
As a method for growing a silicon single crystal, the Czochralski method is well known, wherein silicon raw materials are put in a crucible made of quartz and are melted, then a silicon seed crystal is soaked into the liquid silicon and is pulled up while being rotated, thereby growing silicon single crystal at the lower end of the seed crystal.
As silicon raw materials to be put in the crucible in the first place, lumps of shattered poly crystal are often used. In consideration of the decrease of volume with the melting, at first, lumps of the material are heaped in the crucible. As the melting proceeds, lumps of the material become engaged in the form of a bridge. Accordingly, there is a problem in that the bridge collapses to scatter the liquid silicon, or the quartz is warped.
In order to solve the difficulties mentioned above, silicon granules are used as silicon raw materials (to be described as the initial charge silicon materials) to be put in the crucible in the first place. In the case where the silicon granules are used as the initial charge silicon materials, the difficulties described above are solved. Moreover, since the rate of filling can be made larger as compared with that of the lump material, cost per chip can be reduced. It also is easily dealt with, for its volume is smaller compared with that of the lump material.
When the silicon materials put in the crucible are melted, the volume is decreased. In order to make the best use of cubic contents of the crucible, additional silicon materials (to be described as charge up silicon materials) are supplied into the crucible and melted. As the methods for supplying charge up silicon materials, a method for supplying and melting cylinder shape silicon materials (Japanese Utility Model Application Publication No. 50-11788, 1975) or a method for supplying and melting lump silicon materials by using a special tool (Japanese Patent Application Laid-Open No. 50-11788, 1975) is well known.
In the case where the cylinder shape silicon materials are used, there are difficulties in that the crucible is broken down and the liquid silicon is leaked when too much of the materials are put in and in that the liquid silicon is boiled when the materials and the liquid silicon are left alone separately. Also, in the case where the lump silicon materials are used, since the liquid silicon is scattered when the materials are supplied into the liquid silicon, the surface of the liquid silicon is temporarily solidified and after that, the materials are supplied. But there is a in that the crucible is broken down and the liquid silicon is leaked during solidification.
In order to solve the difficulties described above, sometimes silicon granules are used as charge up silicon materials. In the case where silicon granules are used for the purpose, this difficulties described above are solved.
In order to grow the silicon single crystal successively while a seed crystal is pulled up, it is necessary to supply the silicon materials into the crucible corresponding to the growth volume of the single crystal, since the crucible volume has its own limit. Also the supply of the silicon materials (to be described as additional charge silicon materials) must be carried so as not to change the growth condition.
For this reason, in the conventional example, by providing inside of the crucible another crucible or a cylinder having an opening for flowing the liquid silicon, the surface of the liquid silicon can be divided into two regions including an inner region for pulling up the single crystal and an outer region for supplying silicon materials (Japanese Patent Application Laid-Open No. 57-183392, 1982 and No. 47-10355, 1972). These methods intend to reduce as much as possible the effect of wave motion, dust, temperature change and so on of the surface of the liquid silicon, accompanying the supply of the materials, exerted upon the inner region as the crystal growth region. Also as the additional charge silicon materials, lump shaped silicon materials which are the broken poly crystal silicon are widely used.
Since the shape of the lump silicon materials descrbed above is not fixed, there is a difficulty in that the material supplying portion is easy to get clogged as the materials become engaged in the state of a bridge. Hereupon, in order to solve the difficulty, sometimes silicon granules are used as additional charge silicon materials.
In the case where the silicon granules are used for initial charge and charge up, however, there is a problem in that scattered matter accumulates on the surface of the crucible due to the explosion phenomenon which occurs when the silicon materials are heated up to immediately before the melting temperature, and the accumulated matter drops on the surface of the liquid silicon, disturbing the crystal growth condition to produce faulty crystal.
Furthermore in the case where the silicon granules are used as additional charge, materials scattered matter drops into the inner region (the single crystal growth region) or onto the crystal growth surface due to the same explosion, disturbing the crystal growth condition to induce faulty crystal.